Recently, a new television system which offers to viewers an improved television picture has been researched. The system will be referred to as an improved picture television system hereafter. The improved picture television system will provide an improved picture resolution and/or a wide-screen picture. For example, an improved definition television system (referred to as an IDTV system hereafter) has been developed as one of the improved picture television systems.
Such an improved picture television is intended to have a compatibility with current television receivers widely used according to a prescribed current television system, e.g., the NTSC system, the PAL system and the SECAM system. This is because it is desirable that programs transmitted by the improved picture television system can be received by television receivers for both the current television system and the new system.
In the improved picture television system, e.g., the IDTV system, following two items have been particularly researched for improving the television picture.
(1) Aspect ratio (the ratio of the width to the height of a displayed picture) for providing a wide-screen picture: PA0 (2) Horizontal resolution for improving the picture resolution: PA0 (1) The picture improving signal is transmittable for both the dynamic picture and the static picture; PA0 (2) The picture improving signal gives little disturbance to conventional television receivers; and PA0 (3) Wide-screen television receivers can receive the picture improving signal at a sufficient S/N.
A current color television system, such as the NTSC color television broadcasting system has a 4:3 aspect ratio and a horizontal resolution of 330 TV-lines. The NTSC color television broadcasting system is adopted in the United States, Japan and elsewhere. The NTSC system has been appraised as a color television broadcasting system compatible with the monochrome television broadcasting system and has a sufficient performance for color television broadcasting.
As to an improvement of the aspect ratio, some proposals are now presented. For example, a 5:3 aspect ratio, a 6:3 aspect ratio, a 16:9 aspect ratio, etc. are proposed.
As to an improvement of the horizontal resolution, the horizontal resolution of the current system is limited up to 330 TV-lines due to the transmission frequency band specified by the NTSC standard is limited to 4.2 MHz. While, a vertical resolution of the current system is about 450 TV-lines (effective resolution is 480 TV-line, but about 30 TV-lines are over-scanned). Therefore, it is desirable to increase the horizontal resolution from a viewpoint of balance to the vertical resolution.
A system which aims to improve both the above two items, i.e., the aspect ratio and the horizontal resolution, and has a compatibility with current television receivers is proposed, e.g., in the article entitled "A Compatible High-Definition Television System (SLSC) with Chrominance and Aspect Ratio Improvements" by Joseph L. LoCicero et al., SMPTE Journal, May 1985.
Referring now to FIG. 1, the SLSC (abbreviation of Split Luminance, Split Chrominance) system will be briefly described below. FIG. 1 shows a frequency spectrum diagram of composite signal according to the SLSC system. In FIG. 1, the frequency band 0-4.2 MHz carries a signal having a compatibility with current NTSC system television receivers (the signal will be referred to as a current television picture signal hereafter). The frequency band 4.9-10.1 MHz carries an additional signal for improving the television picture (the signal will be referred to as a picture improving signal hereafter), i.e., for increasing the aspect ratio and resolutions of luminance and chrominance. That is, the extra frequency band 4.9-10.1 MHz is used for transmitting the picture improving signal in the SLSC system.
According to the SLSC system, the lower frequency band 0-4.2 MHz carries the current television picture signal but does not carry the picture improving signal. Therefore, the SLSC system has not only a compatibility but also a high disturbance rejectivity. However, the SLSC system using such a wider transmission frequency band is not effective for channel allocation. Especially, the SLSC system is not acceptable for broadcast service areas where a lot of channels are condensed in a limited television broadcasting frequency band. Furthermore, most broadcasting facilities used for the current television system are not equipped for the transmission of such a wide frequency band of up to 10 MHz. Therefore, current broadcasting television facilities must be replaced by new wide frequency band facilities for the SLSC system at a huge cost.
Therefore, it is desirable to transmit both a current television picture signal and a picture improving signal by using only one channel frequency band, i.e., 0 to 4.2 MHz. Such a system capable of transmitting the signals by the frequency band 0 to 4.2 MHz will have a good compatibility with current broadcasting facilities, such as transmitters and video tape recorders.
To transmit both a current television picture signal and a picture improving signal with one channel frequency band, a signal multiplexing system has been also researched. For example, such a multiplex system is proposed in the article entitled "Extended Definition TV Fully Compatible with Existing Standards" by T. Fukinuki et al., IEEE Transaction on Communications, Vol. COM-32, No. 8, August 1984.
Referring now to FIG. 2, the multiplex system will be briefly described below. FIG. 2 shows a temporary frequency to vertical frequency co-ordinate diagram representing a television signal according to the multiplex system. According to the multiplex system, a detail signal component YH of a luminance signal (the luminance signal component of frequency band 4 to 6 MHz, the component YH, will be referred to as a high frequency luminance signal hereafter) is multiplexed to a specific spectrum region of a current television signal according to the NTSC system. The specific spectrum region exists in small diamonds YH in the first and third quadrants of the co-ordinate diagram, as shown in FIG. 2. On the other and, a chrominance signal C exists in two other small diamonds C in the second and fourth quadrants.
The small diamonds YH are left unused in a case of static picture, but the small diamonds YH are filled in a case of dynamic picture. Thus, the multiplex system can be adopted for improving definition of picture in the case of static picture.
On the other hand, a television system for the increased aspect ratio picture (the system will be referred to as a wide-screen television system hereafter) must transmit the picture improving signal in either of the static picture or the dynamic picture. Therefore, such a multiplex system proposed in the "Extended Definition TV Fully Compatible with Existing Standards" is difficult to be adopted for the wide-screen television system. For adopting the multiplex system to the wide-screen television system, a dynamic picture component must be depressed or limited. However, the depression of the dynamic picture component will damage a natural movement of the picture. As a result, the multiplex system is less compatible for current television receivers.
The high frequency luminance signal component YH of a general television picture is very low in level in comparison to a low frequency luminance signal component YL. Thus, the high frequency luminance signal component YH is less harmful for the current television receivers, and the component YH is multiplexed for improving the definition of the picture. On the other hand, the wide-screen television system is required to transmit a relatively high level picture improving signal. This is because the picture improving signal for the wide-screen television system includes not only the Low level high frequency luminance signal component YH but also the high level low frequency luminance signal component YL. If the picture improving signal is transmitted after a level suppressed, the received picture has a decreased S/N (signal to noise) ratio.
Therefore, a transmission of the picture improving signal by using the conventional television signal band of 4.2 MHz requires that;
However, there has been developed a system that can meet all of the requirements.